It is common practice in many commercial and military jet aircraft to brake the aircraft upon landing by effectively reversing the thrust of the jet engines. The most common arrangement for providing such thrust reversal is to provide a pair of clam shell shaped members pivoted within the engine housing to swing from a position eclipsing side openings in the engine to a position intercepting the thrust gases normally passing rearwardly of the engine thereby directing the gases laterally out of the side openings. Simultaneously with the swinging movement of the clam shell structures, doors or partitions normally covering the side openings swing upwardly and serve as a deflection or baffle wall for further guiding the laterally deflected thrust gases forwardly thus providing the desired reverse thrust action.
The provision of the swinging door arrangements for cooperation with the clam shell structures in providing thrust reversal represents a substantial additional expense as well as an increase in weight of the overall aircraft. Moreover, there are many mechanical and hydraulic accessories necessary to operate such doors and as a consequence careful and periodic maintenance is required to assure reliability and safety.
More recently, it has been proposed and actually placed in operation, the provision of a grid-like vane structure fitted within the opening normally exposed when the clam shell structures pivot to deflect the thrust gases laterally. The vane structure is used in place of a swinging door structure and is a fixed installation; that is, there are no moving parts. The vanes in the grid structure are tilted forwardly slightly to provide the necessary forward component of gas movement necessary to realize a thrust reversal reaction. When the clam shell structures are in their normal inoperative position, they simply underlie the grid-like vane structure.
The foregoing proposed vane structures have greatly decreased maintenance cost, increased reliability and decreased the overall weight of the engines as compared to the prior swinging door type arrangements. On the other hand, there have been encountered numerous difficulties with the vane structures presently proposed. More particularly, the presently proposed and used vane structures include a plurality of beams or "strong backs" as they are referred to in the art extending generally in parallel vertical planes. Between these beams there are welded small individual vanes extending transversely between adjacent beams. The vanes are generally staggered in order that the ends of each of the individual vanes can be properly welded to the inside opposed surfaces of the beams. This staggered arrangement is necessary in order that a chill block may be backed against the opposite side of the beam to which one end of a vane is to be welded. If the vanes were in alignment in a transverse direction, proper welding could not really be effectively carried out.
The process or method of assembling the vane structure is thus time consuming and expensive and further, the final welds themselves are not always reliable and tend to develop cracks.
As a further consequence of the foregoing type of assembly, it will be evident that each of the individual vanes welded into position must carry a load or constitute a load bearing element when deflecting the thrust gases to provide the reverse thrust. Thus, unless each and every one of the vanes are properly welded in position, one or more may work loose or be blown out. Particular trouble has been encountered in those vanes along the sides of the vane structure; that is, those vanes positioned between the first and second beams from each side or those vanes furthest from the center of the grid. It is believed that one reason for the greater weakness in these latter mentioned vanes is the fact that the chord length of the vanes is greater at the sides than towards the center of the vane structure as a consequence of the overall convex curved surface in which the vane structure lies to conform to the circumferential extent of the side opening of the engine.